Thermal transfer printing has displaced impact printing in many applications due to advantages such as relatively low noise levels and high reliability which are attained during the printing operation. Thermal transfer printing is widely used in special applications such as in the printing of machine-readable bar codes and magnetic alpha-numeric characters. The thermal transfer process provides great flexibility in generating images and allows for broad variations in style, size and color of the printed image. Thermal transfer printing requires a special medium for transferring ink or other sensible material to a receiving substrate. This special medium, referred to herein as a "thermal transfer medium," typically comprises a functional layer on a substrate. The functional layer, also referred to as a "thermal transfer layer," contains the ink or sensible material that is transferred upon application of heat from a thermal print head. The thermal transfer layer comprises a mixture of components which vary significantly in identity and concentration, depending on the end use. Representative documentation in the areas of thermal transfer printing and coating formulations for thermal transfer layers include the following:
U.S. Pat. No. 3,663,278, issued to J. H. Blose et al. on May 16, 1972, discloses a thermal transfer medium comprising a base with a coating comprising of cellulosic polymer, thermoplastic aminotriazine-sulfonamide-aldehyde resin, plasticizer and a "sensible" material such as a dye or pigment.
U.S. Pat. No. 4,315,643, issued to Y. Tokunaga et al. on Feb. 16, 1982, discloses a thermal transfer element comprising a foundation, a color developing layer and a hot melt ink layer. The ink layer includes heat conductive material and a solid wax as a binder material.
U.S. Pat. No. 4,403,224, issued to R. C. Winowski on Sep. 6, 1983, discloses a surface recording layer comprising a resin binder, a pigment dispersed in the binder, and a smudge inhibitor incorporated into and dispersed throughout the surface recording layer, or applied to the surface recording layer as a separate coating.
U.S. Pat. No. 4,463,034, issued to Y. Tokunaga et al. on Jul. 31, 1984, discloses a heat-sensitive magnetic transfer element having a hot melt or a solvent coating.
U.S. Pat. No. 4,628,000, issued to S. G. Talvalkar et al. on Dec. 9, 1986, discloses a thermal transfer formulation that includes an adhesive-plasticizer or sucrose benzoate transfer agent and a coloring material or pigment.
U.S. Pat. No. 4,687,701, issued to K. Knirsch et al. on Aug. 18, 1987, discloses a heat sensitive inked element using a blend of thermoplastic resins and waxes.
U.S. Pat. No. 4,707,395, issued to S. Ueyama et al., on Nov. 17, 1987, discloses a substrate, a heat-sensitive releasing layer, a coloring agent layer, and a heat-sensitive cohesive layer.
U.S. Pat. No. 4,777,079, issued to M. Nagamoto et al. on Oct. 11, 1988, discloses an image transfer type thermosensitive recording medium using thermosoftening resins and a coloring agent.
U.S. Pat. No. 4,778,729, issued to A. Mizobuchi on Oct. 18, 1988, discloses a heat transfer sheet comprising a hot melt ink layer on one surface of a film and a filling layer laminated on the ink layer.
U.S. Pat. No. 4,923,749, issued to Talvalkar on May 8, 1990, discloses a thermal transfer ribbon which comprises two layers, a thermosensitive layer and a protective layer, both of which are water based.
U.S. Pat. No. 4,975,332, issued to Shini et al. on Dec. 4, 1990, discloses a recording medium for transfer printing comprising a base film, an adhesiveness improving layer, an electrically resistant layer and a heat sensitive transfer ink layer.
U.S. Pat. No. 4,983,446, issued to Taniguchi et al. on Jan. 8, 1991, describes a thermal image transfer recording medium which comprises as a main component, a saturated linear polyester resin.
U.S. Pat. No. 4,988,563, issued to Wehr on Jan. 29, 1991, discloses a thermal transfer ribbon having a thermal sensitive coating and a protective coating. The protective coating is a wax-copolymer mixture which reduces ribbon offset.
U.S. Pat. Nos. 5,128,308 and 5,248,652, issued to Talvalkar, each disclose a thermal transfer ribbon having a reactive dye which generates color when exposed to heat from a thermal transfer printer.
And, U.S. Pat. No. 5,240,781, issued to Obata et al., discloses an ink ribbon for thermal transfer printers having a thermal transfer layer comprising a wax-like substance as a main component and a thermoplastic adhesive layer having a film forming property.
To be suitable for thermal transfer printing, there are many requirements placed on the thermal transfer layers and coating formulations which produce them. For example, the properties of the thermal transfer layer must permit transfer from a carrier to a receiving substrate and provide a stable, preferably permanent image. The properties needed to meet these requirements are in conflict and require a mixture of components to address both needs, typically a wax to provide softening characteristics for transfer and a thermoplastic polymer resin to provide stability and resistance to handling of the image after transfer. Conventional thermal transfer (coating) formulations have employed organic solvents to solubilize or emulsify the dry components for deposition on a substrate. The use of organic solvents complicates compliance with environmental regulations and restrictions. The use of solvent also adds to the cost in that the solvent is removed from the coating and captured or incinerated.
Water-based and water-rich coating formulations have recently been developed to improve safety, reduce costs, and simplify compliance with environmental regulations and restrictions. For example, U.S. Pat. No. 4,923,749 issued to Talvalkar, discloses a thermal transfer ribbon which comprises a thermal sensitive layer and protective layer, both of which are water-based. Extensive work has been done to develop water-rich systems to replace organic solvent-based systems. In these formulations, both the waxes and resins must be soluble, dispersible or emulsifiable in water, which limits the selection. Suitable waxes and resins are available in separate aqueous emulsions. However, in preparing a water-rich/water-based coating formulation, the selection of components is further limited in that it is necessary to find a wax emulsion which is compatible with the resin emulsion to avoid precipitation. To achieve some combinations it is necessary to incorporate organic solvents to prevent precipitation of the polymer resin.
Alternatives to using solvent are hot melt techniques wherein a solid coating formulation is melted, applied to a substrate, and chilled to resolidify the formulation. These hot melt techniques are not well suited for multilayer techniques in that the first layer (subcoat) can be compromised when a second layer (top coat) is applied at high temperatures.
It is desirable to prepare thermal transfer layers from a coating formulation which does not require any solvent, whether aqueous or organic, and does not require the high temperatures of hot melt techniques for application to a substrate.
Ultraviolet radiation curable inks are known and most comprise a reactive oligomer, a reactive monomer, a photoinitiator, a pigment and optional additives. UV curable inks are commonly used in printing methods other than thermal transfer printing, such as screen printing, and lithography techniques for printed circuit boards, examples being described in U.S. Pat. Nos. 5,200,438, 5,391,685, 5,270,368, 4,680,368 and 5,500,040. A UV curable ink said to be suitable for ink jet printing is described in U.S. Pat. No. 4,258,367. Conventional UV curable inks typically do not have the transfer properties necessary for use in conventional thermal transfer printing processes with conventional thermal transfer printers after cure. They are typically formulated for use in printing methods wherein curing provides a permanent image.